1. Field of the Invention
The present invention generally relates to an air-fuel ratio control system and an air-fuel ratio control method of an internal combustion engine. More particularly, the present invention relates to an air-fuel ratio control system and an air-fuel ratio control method of an internal combustion engine, which perform feedback control of an air-fuel ratio of an air-fuel mixture using an air-fuel ratio sensor.
2. Description of the Related Art
Conventionally, in order to reduce an HC amount and an NOx amount in exhaust gas, and ensure an adequate driving power of an internal combustion engine, it is required that an air-fuel ratio of an air-fuel mixture be controlled to reach a desired value (target value) for each running range of the engine. To implement this, Japanese Laid-Open Patent Application Publication Nos. Sho. 57-105530 and 2006-226234 disclose a technique (O2 feedback control) for performing feedback control of an air-fuel ratio of an air-fuel mixture by controlling an injector using an O2 sensor provided in an exhaust system.
An output characteristic, for example, an output voltage of the O2 sensor changes when an air-fuel ratio becomes closer to a theoretical air-fuel ratio. It can be detected only whether the air-fuel mixture is lean or rich by monitoring the output of the O2 sensor, and therefore, a value of the air-fuel ratio cannot be detected. For this reason, the conventional O2 feedback control uses a method which gradually changes a feedback compensation coefficient for making compensation for a fuel feed amount while monitoring whether the output of the O2 sensor indicates a lean air-fuel ratio or a rich air-fuel ratio. Unlike normal feedback control, setting the feedback compensation coefficient according to a deviation is unfeasible. As a result, the air-fuel ratio which can be achieved during the O2 feedback control is limited to a value close to the theoretical (stoichiometric) air-fuel ratio at which the output characteristic of the O2 sensor changes in a manner that can be used for feedback control.
However, an air-fuel ratio in a certain running range of the engine is set to a value other than the theoretical air-fuel ratio. Particularly, in straddle-type vehicles such as a motorcycle, to ensure a level of engine driving power output or to protect an engine, various target air-fuel ratios are set to correspond to running ranges of the engine, respectively. Therefore, a running range in which the O2 feedback control is enabled is especially limited. If the control method in which the feedback compensation coefficient is changed gradually is used in such a case, an actual air-fuel ratio cannot become the desired air-fuel ratio at a time when the O2 feedback control is initiated, or when a compensation amount required in the O2 feedback control changes rapidly because of an abrupt change in the running range. As a result, the quality of exhaust gas is degraded, or the engine driving power becomes less than desired. This is a first problem associated with a conventional method.
If an air-fuel ratio realized by the control is limited to the theoretical air-fuel ratio, the quality of exhaust gas may be degraded, or the engine driving power may become less than desired. In non-O2-feedback control mode, in a running range in which an air-fuel ratio other than the theoretical air-fuel ratio is required to, for example, ensure a level of engine driving power output or to protect the engine. This is because, for example, it is impossible to make compensation for a fuel amount if the air-fuel ratio is deviated from a desired value due to degradation of the engine.
By comparison, Japanese Laid-Open Patent Application Publication No. 2006-226234 discloses a technique in which engine operation control is executed so that the air-fuel ratio reaches a value closer to the theoretical air-fuel ratio in such a manner that compensation is made for a base fuel amount by changing the feedback compensation coefficient, while monitoring an output of an O2 sensor by O2 feedback control, the feedback compensation coefficient at a time point when the air-fuel ratio reaches the value closer to the theoretical air-fuel ratio is learned, and compensation is made for the base fuel amount, using the learned feedback compensation coefficient, in a non-O2-feedback control mode.
In the conventional technique in which the feedback compensation coefficient is learned during the O2 feedback control, the feedback compensation coefficient is affected by a deviation between a target air-fuel ratio and the theoretical air-fuel ratio, in addition to a deviation of the air-fuel ratio due to degradation of the engine or the like, because a target air-fuel ratio changes over time according to the running state in an engine in which target air-fuel ratios are set to respectively correspond to the running states, like an engine in a straddle-type vehicle. Therefore, it is difficult to correctly detect only the deviation of the air-fuel ratio due to degradation of the engine or the like, based on the feedback compensation coefficient, and to prevent degradation of the quality of exhaust gas or occurrence of an undesired driving power output characteristic in the non-O2-feedback control mode, in the non-O2-feedback control mode. This is a second problem associated with the conventional method.